Nitro acetal propellants



United States 3,7l,5 98 Patented Jan. 1, 1963 lice 3,071,598 NlTRtl) ACETAL l'ilRUPELLANT Henry B. Haas, Gustave Bryant Bachrnan, and Henry Feuer, West Lafayette, End, and Kenneth S. Warren, Morris Plains, NJL, assignors to Purdue Research Foundation, West Lafayette, End, a corporation of Indiana N Drawing. Filed June 1, 1943, Ser. No. 30,514 4- Claims. (Cl. 260347.8)

The present invention relates to solid propellants and to combustible components thereof. More particularly, the invention relates to smokeless propellants embodying nitro acetals, especially novel polymeric nitro acetals having unique characteristics which allow their use as the major thrust-producing component of such solid smokeless propellants of the type utilized in rocket and other similar jet-propulsion type motors demanding great power.

Since the rapid development of jet-type motors, a great demand has arisen for solid propellants which may be used to provide impelling force in such motors. Because of the many exacting specifications which such a propellant must fulfill, very few, if any, solid fuels having suitable characteristics have been available up to the present time.

An ideal solid propellant would exhibit the following characteristics:

1) It should be solid and stable over the range of ambient temperatures of -40 to +60 degrees Centigrade and under pressures between 300 and 1500 pounds per square inch.

(.2) It should burn uniformly and have a low temperature exponent, i.e., its burning rate increase with temperature should be as small as possible.

(3) It should have a low pressure exponent, i.e., low change in burning rate with pressure variation.

(4) It should be capable of being shaped into large grains and should preferably be composed-of large molecules.

(5) It should be substantially smokeless.

(6) it should have a satisfactory oxygen balance, i.e., it should possess enough oxygen to burn all carbon to carbon monoxide and one-third of the hydrogen to water.

(7) It should not undergo deterioration upon storage.

('8) It should possess a minimum of susceptibility to detonation under conditions of employment and should be stable upon heating.

(9) It should not be hygroscopic.

(10) It should have a high specific impulse.

Still other specifications are desirable, but these may be considered sub-specifications of those enumerated above.

It has previously been proposed to use as solid fuels compositions embodying cellulose nitrate, but with such compositions the temperature coefiicient is undesirably high so that the rate of burning of the fuel is relatively slow when cold and quite rapid when hot. While the rate at intermediate temperatures is satisfactory, it is impossible to maintain such desirable temperatures for any extended period. Further, cellulose nitrate is inherently unstable, and thus fails to fulfill another very important requirement.

The disadvantages of nitrocellulose compositions have been partially overcome with the provision of compositions embodying ethyl cellulose-cas'tor oil, neoprene casting cements, cross-linked maleic anhydride-styrene resins, or other styrene-linear polyester resins and peptized gums in admixture with perchlorate powders. However, with perchlorates, shorting of electrical equipment and corrosion is commonly experienced, and the white potassium chloride smoke which comprises approximately 57 percent of the exit gases when potassium chlorate is used limits visibility to an undesirable and hazardous extent. Likewise, when ammonium perchlorate is employed, the misttorming hydrogen chloride present in the exhaust gases is objectionable for the same reasons.

it is an object of the present invention to provide chemical compounds suitable for incorporation into a solid propellant or fuel. It is a further object to provide solid propellants incorporating the said compounds which are stable over a wide range of ambient temperatures, substantially smokeless, in good oxygen balance, having a low temperature coeflicient, and not undesirable from other standpoints. Another object of the invention is the provision of such propellant compositions incorporating plasticizers and/ or fillers to render the compositions even more suitable for the prescribed use. Other objects of the invention will become apparent hereinafter.

The compounds useful in preparing the propellants are the condensation products or" an alcohol, e.g., polyvinyl alcohol, and a nitro aldehyde, and especially polymeric nitro acetals having a minimum oxygen balance of minus 80. Representative compounds are the polyvinyl acetals of 2,4,6-trinitrobenzaldehyde, 5-nitrofurfural, 2,3,3-trinitropropa'nal, 2,5-dinitrofurfural, 2,4,6 tris (2',2,2'-trinitroethyl)-benzaldehyde, and the like. Besides polyvinyl alcohol may be employed other suitable alcohols such as methanol, ethanol, propanol, 2,2-dinitro-1,3-propanediol, 2,2-dinitropropanol, and 2,4,6-trinitrophenylethanol. In cases where the alcohol is not polymeric, the nitro acetal must be employed in combination with other compositions such as those aforementioned as not completely satisfactory propellants. However, even with such admixtures of nitro acetal with cellulose nitrate-nitroglycerine mixtures, styrene-maleic anhydride resins, peptized gums, ethyl cellulose-caster oil compositions, et cetera, together with solid inorganic oxidizers, considerable advantage of oxygen balance, smokelessness, burning rate and thrust is realized over the simple compositions not having a nitro acetal incorporated therein.

The starting aldehydes and methods for their preparations are already known. Trinitrobenzaldehyde has been prepared by Sachs and Everding, Berichte 35, 1236 (1902); ibid. 36, 999 (1903) and by Secareanu, Berichte 64, 836 (1931). The 2,3,3-trinitropropanal dipotassium salt may be prepared from mucobromic acid (from bromine and furoic acid or furfural, Hill, A. Chem. Jour. 3, 4 (1881)) according to the procedure of Torrey, A. Chem. Jour. 24 457 (1900). Nitrofurfural may be prepared by the procedure of R. Marquis (Comp. rend. 132, -142 (1901); ibid. 134, 776-777 (1902); Br. Chem. Ab. 80, I, 222 (1901); ibid. 82, I, 483 (1902) or Gilman and Wright, J. Am. Chem. Soc. 52, 2550-2554, 4165- 4166 (1930). Aromatic nitro aldehydes may also be prepared by the oxidation of corresponding methylated hydrocarbon derivatives according to the procedure of Thiele and Winter, Annalen 311, 353 (1900). The method of Organic Synthesis, Coll. vol. II, p. 442, John Wiley and Sons (1943) is also applicable to the preparation of nitro aldehydes in general. Methods for synthesis of the alcohols are known and in some cases the alcohols are commercially available.

Representative nitro acetals which may for example be used in preparing propellants according to the present invention are the methyl, glycol, glycerol, mannitol, sorbitol, erythritol, pentaerythritol, and other acetals of nitro aldehydes such as, for example, nitrofurfurals, diand trihitrobenzaldehydes, 2,3,3-trinitropropanal, and the like. While the above alcohols are monomeric, the preferred alcohol for condensation with a nitro aldehyde to produce the nitro acetal is polymeric, such as polyvinyl alcohol. This is because, as aforesaid, the monomeric nitro acetals,

'ing to known procedure for such a condensation. example, the selected alcohol may be dissolved or suswhile adapted for use in conjunction with the other powder-base inorganic oxidizer compositions (such as (a) ethyl cellulose, rubber, ammonium picrate, and potassium nitrate, (b) double base powder, potassium perchlorate, (c) ammonium nitrate, ammonium picrate, peptized rubber, or (d) any of the compositions mentioned previously as not being entirely satisfactory propellants) are not generally satisfactory as the major combustible component of a propellant, while the polymeric nitro acetals are especially adapted for use in such capacity. Only nitro acetals having a minimum oxygen balance of minus 80 are useful in the novel propellant compositions.

Oxygen balance of a combustible compound may be calculated readily. A compound is considered to be in perfect oxygen balance when it contains sufiicient oxygen to burn all the carbon to carbon dioxide and all of the hydrogen to water. It is then said that the compound has an oxygen balance of zero, the value being determined by inserting the values in the formula:

O X 100 R 100 oxygen balance quired.

(3) The formula thus reads A compound having a minimum oxygen balance of -50 is considered entirely suitable. This calculation is based on the assumption that a compound containing sufiicient oxygen to burn all the carbon to carbon monoxide and one-third of the hydrogen to Water will be productive of substantially no smoke. Likewise, it is considered that a propellant having a minimum oxygen balance of 80 is suitable for all practical purposes, and experimental tests have proved the correctness of this assumption. At any greater negative value, the increased amount of smoke produced, and decreased thrust per weight of fuel, makes use of the propellant hazardous and undesirable. Therefore, it is necessary that the nitro acetal component of the propellant have a minimum oxygen balance of -80. If monomeric, it can thus be used as a filler, additive, or plasticizer to bring the propellant composition into proper oxygen balance. If polymeric, it is necessary that the oxygen balance be within the prescribed range so that it may be used as the major combustible component of the smokeless propellant. It is a simple matter to calculate the total oxygen balance for a composite propellant composition by use of the above formula.

The nitro acetals may be prepared by condensation of a selected nitro aldehyde with a suitable alcohol accord- For pended in a suitable medium, such as glacial acetic or propionic acid, and the aldehyde, also dissolved in a solvent such as glacial acetic acid, added thereto. Higher viscosity alcohols ordinarily appear to produce the most desirable type of product. A suitable acid catalyst, such as hydrochloric acid, should preferably be present during the condensation reaction to effect a more rapid rate of reaction and ready attainment of a higher molecular weight polymer. After heating at a suitable reaction temperature, e.g., sixty degrees centigrade, preferably with agitation, the condensation is complete and reaction may be discontinued and desired product separated.

If the alcohol is polymeric, such as polyvinyl alcohol, the reaction products may be recovered by quenching or drowning in cold water, whereafter the precipitate may be separated, Washed with water, and dried. Such polymeric nitro acetals usually have a softening point or susceptibility to plasticization which allows them to be molded or cast in a suitable matrix and plasticized, if desired, with suitable plasticizers.

The novel propellants of the present invention comprises a nitro acetal, such as those mentioned previously, having a minimum oxygen balance of minus and at least one additional component selected from solid combustible oxygen-containing plasticizers, fillers, additives, and oxidizers.

The polymeric nitro acetals may, for example, be plasticized with compounds which are also in satisfactory oxygen balance so that the plasticized product falls within the prescribed range. Compounds which may be incorporated with the polynitro acetals as plasticizers or additives, and which are suitable for such capacity are o-nitrotoluene, 1,1,2,2-tetranitroethane, 2,2-dinitropropanol, nitromethane, nitroform, tetranitromethane, methyl nitroacetate, glycol nitroaceate, glycol dinitrate, glycerol trinitrate, mannitol hexanitrate, 2,2,3,3-tetranitrobutane,

' 2,3,3-trinitroisopentane, Z-methyl 2,3,3 trinitropentane,

2,3,3-trinitroisohexane, nitroguanidine, nitrourea, and the like. The employment of certain polynitro alkanes in propellant compositions is more fully disclosed and claimed in application Serial No. 30,513, filed concurrently herewith. Organic plasticizers or additives other than those mentioned above, such as guanidine or urea derivatives, dibutyl-phthalate, et cetera, may also be employed, providing that the relative quantities of nitro acetal and plasticizer are chosen so that the plasticized composition is still in proper oxygen balance.

When the thermoplastic nitro acetals are plasticized or admixed with the above or similar nitro organic compounds, determinations on the polymer indicate a low burning-law exponent. This is very important as indicative of a low pressure and temperature sensitivity, which, as mentioned above, is highly desirable in a solid propellant of the type here concerned.

Moreover, it has been found that, if desired, a solid inorganic oxidizer such as ammonium nitrate, potassium nitrate, or potassium perchlorate, may be incorporated into the polymeric nitro acetal and plasticization or intimate admixture accomplished subsequently thereto. When such procedure is followed, the plasticized or filled nitro acetal and oxidizer composition is still of a very desirable nature, exhibiting a burning-law exponent only slightly higher than that of the nitro acetal polymer itself. By incorporation of such an inorganic oxidizer into the nitro acetal, it is, for example, possible to use as plasticizer compounds other than the nitro compounds listed above, if desired, making up the lack of oxygen balance in the composition through employment of the selected inorganic oxidizer. Compounds such as nitroglycerine or ethylene glycol dinitrate may also be used to obtain a more favorable oxygen balance, if desired, and perchlorates are preferably avoided, if possible. The procedure for calculating burning-law exponents or temperature coeflicients is known (Crawford and Huggett, OSRD Report 4009; see also OSRD Report 5577, p. 52). This procedure allows indirect evaluation of the temperature coefficient of a fuel by the experimental measurement of burning-rate change with respect to pressure and temperature. Assuming that the Paul Vielle equation proposed by the French physicist in 1893 holds,

r=cP

where r is the linear burning rate of a powder, c and n are constants for a certain composition and P is gas pressure.

Ther d log P and n may therefore be determined by estimating the slope of a straight line obtained by plotting log r against log P.

Desirable temperature coefi'icients are indicated by low values of n as indicated by the relation 0! log P 1 d log 1" I: dT :l '1 n dT 1 where T equals absolute temperature and area of burning surface of propellant K:

cross-sectlonal area of throat As noted from Examples 5, 6, and 7, compositions of the present invention embodying polymeric nitro acetals exhibit a very low temperature coeflicient, evidenced by low values of n, which are generally below 0.60. The data obtained by firing these compositions in a Crawford bomb corresponds very closely to those obtained in actual firing tests in midget motors.

The following examples are illustrative only and are in no way to be construed as limiting.

EXAMPLE 1 Polyvinyl Acetal of T rinitrobenzaldehyde -OH2CHCH2CH OOH-O OZN N02 It follows that:

(a) To a suspension of 0.80 mole (70.4 grams) of high viscosity polyvinyl alcohol in 600 milliliters of glacial acetic acid there was added one mole (241 grams) of 2,4,6-trinitrobenzaldehyde dissolved in 1000 milliliters of glacial acetic acid. An acid catalyst, consisting of milliliters of water, was added with stirring and the reaction continued for 65 hours, while the reaction temperature was maintained at about 60 degrees centigrade. At the end of the reaction time the transparent solution was dropped into about ten gallons of water with vigorous stirring. The precipitated fibrous nitro acetal was filtered, washed with one percent sodium carbonate and then with water. The nitro acetal resin Was obtained in a yield of about 80 percent and was yellow in color, combustible and had a softening point of 85 degrees centigrade.

The polynitro acetal plasticized readily with tetranitromethane, methyl nitroacetate, nitromethane, and o-nitrotoluene.

(b) One-tenth of a mole (24.1 grams) of 2,4,6-trinitrobenzaldehyde was dissolved in 50 milliliters of glacial acetic acid (or other solvent). To 100 milliliters of the same solvent there was added 7.0 grams of polyvinyl alcohol and stirring employed until the alcohol was dissolved or dispersed. This represents a mole ratio of the aldehyde to the alcohol of 1.25, where a mole of polyvinyl alcohol is considered as CH2-CHCH2-CH (|)H OH The two solutions were transferred to a 300-1nilliliter wide-mouth Florence flask equipped with a stirrer and placed in a 55 degrees centrigrade thermostat. After the' desired amount of dilute hydrochloric acid (equal parts by volume of water and concentrated hydrochloric acid) was added, the mixture was allowed to react with stirring for the desired period of time (see table, below). At the end of the reaction time the contents of the flask were added dropwise to four liters of Water with very vigorous agitation at room temperature. The plastic formed im mediately and was separated from the aqueous solution by filtration. The product was washed with dilute sodium bicarbonate and then water until the pink color, due to the action of sodium bicarbonate, had disappeared. The white plastic mass was finally dried at 60 degrees centigrade. The conditions under which the condensations were conducted are as follows:

' Beac- Moles Vol. Run No. Temp., Viscosity Solvent tion 'INB H01 C. of PVA time, Moles added,

Hrs. PVA ml.

55 Lowacetic acid 5.25 1.25 4 55 do. do 7.0 1. 25 4 70 d0 methanol. 3.0 1.17 11 55 .do aceticacid... 63.0 1.25 4 55 High 70.0 1.25 4 55 Low 70.0 1.25 12 55 Medium l0 70.0 1.25 4

While the runs employing glacial acetic acid as solvent esulted in a clear, brown solution, complete solution of reactants was not obtained in run number 3 Where methanol was used as a solvent.

EXAMPLE 2 Polyvinyl Acetal of 5-Nitrofurfural CH2-CH-CH2CH (a) To a suspension of 0.2 moles (17.6 grams) of high viscosity polyvinyl alcohol in 200 milliliters of glacial acetic acid there was added 0.2 mole (48.60 grams) of S-nitrofurfural diacetate and eight milliliters of six N hydrochloric acid. The mixture was stirred mechanically for hours at 60 degrees centigrade. The clear brown solution was then added dropwise to about two gallons of Water with vigorous stirring. The precipitate was washed with a two percent sodium carbonate solution and then with water. The dried, white polymer burned readily in air and had a softening point of about degrees centigrade.

(b) In a SOC-milliliter round-bottom three-neck flask equipped with a stirrer was placed200 milliliters of glacial acetic acid containing 0.2 mole (48.6 grams) of S-nitrofurfural diacetate. After adding eight milliliters of six N hydrochloric acid and 0.2 mole of high viscosity polyvinyl alcohol which had been emulsified in milliliters of glacial acetic acid, the contents of the flask were maintained at 57 degrees centigrade for 23 hours. At the end of this time the polyvinyl alcohol had completely dis solved. The solution was added dropwise into about ten liters of water with vigorous stirring. The solid product, which immediately formed, consisted of small white balls. The precipitate was washed with two percent sodium carbonate solution, then with water, and dried in an oven at 65 degree centigrade. The dried product burned readily in air. The apparent density, determined by pouring a weighed quantity of the polymer into a graduated cylinder, was 0.12. The product weighed 34 grams, representing about 81 percent of the theoretical yield. The polymer did not liquefy under 200 degrees centigrade with slow rise in temperature.

The nitro acetal polymer plasticized readily with o-ni trotoluene, tetranitromethane, or methyl nitroacetate.

EXAMPLE 3 Polyvinyl Acetal of 2,3,3-Trinitropr0panal -GHaCH-GH2CH O-GHO Twenty grams of the dipotassium salt of 2,3,3-trinitropropanal was added slowly to 100 milliliters of glacial acetic acid. There was a definite decrease in the acidity of the acid upon addition. The mixture was placed in a round-bottom three-neck flask fitted with a thermometer and a stirrer and 4.4 grams of polyvinyl alcohol suspended in 100 milliliters of glacial acetic acid added thereto. The solution was heated to 90 degrees centigrade and refluxed for 48 hours, whereupon the mixture became viscous and was poured into cold water, washed and separated. The nitro acetal which was obtained in this manner burned with an almost smokeless flame and plasticized or formed intimate admixtures with tetranitromethane, 2,2-dinitrpropane, or methyl nitroacetate.

EXAMPLE 4 Fourteen and seventy-six one hundredth grams of the polyvinyl acetal of 2,4,6-trinitrobenzaldehyde and 9.84 grams of dried ammonium nitrate (with 0.1 percent calcium phosphate added) were subjected to tumbling in a ball mill containing 250 grams of stone balls. After 24 hours of blending in the mill, the powdered mixture was plasticized with 3.63 grams of methyl nitroacetate and rolled into a strand, the composition of which was as follows:

Percent Nitro acetal 52.25 Ammonium nitrate 34.83 Methyl nitroacetate 12.92

The strand Was placed on a glass plate at room tempera ture (27 degrees centigrade) to determine weight increase (hygroscopicity) or Weight loss (volatility of methyl nitroacetate). The Weight of the sample and the uniform texture thereof did not change over a period of one month.

EXAMPLE 5 Forty grams of ball-milled polyvinyl acetal of 2,4,6- trinitrobenzaldehyde and sixteen grams of dried ammonium nitrate (0.1 percent calcium phosphate added) were tumbled in the ball mill of Example 4. After 24 hours of blending, the powdered mixture was mixed with tetranitromethane (20.6 grams; of which 6.7 grams volatilized during mixing) and then shaped into strands. The strands were coated with glyptal enamel and air-dried before burning in a Crawford bomb.

Measurement of the burning rate of the strands, containing 57 percent nitro acetal, 23 percent ammonium nitrate, and 20 percent tetranitromethane gave the following data:

Initial Peak Burning Pressure, Pressure, Rate,

p.s.i. p.s.i. Iuches/ Second When graphed, the value of the slope n, using the method of least squares, was found to be 0.57, which is a very low burning-law exponent.

EXAMPLE 6 Sufficient tetranitromethane is added to the desired quantity of pulverized nitro acetal to yield a mixture containing 28 percent thereof. If the mixing is done by hand with a steel spatula, about 40 grams of the nitro acetal worked on a 10 x 10 inch glass plate is convenient. This amount requires 16 grams of tetranitromethane, which is quickly absorbed by the plastic, and the mixture may be finally kneaded with the fingers. A stiff, brown, doughy mass results, which is shaped by rolling on a plate to form strands of whatever length and diameter may be desired.

Composition (a) Initial Peak Burning Pressure, Pressure, Rate,

p.s.i. p.s.i. Inches] Second The burning-law exponent n for this composition was 0.3 6, a very low value.

Composition (b) Initial Peak Burning Pressure, Pressure, Rate,

p.s.l. p.s.i. Inches] Second The burning-law exponent for composition (1)) was r1=0.74.

EXAMPLE 7 In a burning rate experiment similar to that of Example ,6, a strand of 72 percent polyvinyl acetal of 2,4,6-trinitrobenzaldehyde and 28 percent tetranitromethane composition, coated with glyptal enamel, exhibited a burning-law exponent of 0.32.

EXAMPLE 8 Fifteen grams of the dipotassium salt of 2,3,3-trinitropropanal was placed in 200 milliliters of ethanol and anhydrous HCl gas was passed into the solution until all of the salt was converted to the free trinitroaldehyde. The potassium chloride was removed by filtration and the filtrate allowed to stand for several days at room temperature in a stoppered Erlenmeyer flask. Upon testing a portion of the solution, no product was obtained. Fifteen grams of anhydrous CaCl was then added to the remainder of the solution. The solution was allowed to stand for three days at room temperature. Upon evaporating the alcohol, the ethyl acetal of 2,3,3-trinitropropanal was obtained. This compound, as well as other similar nitro acetals, is employed in propellant compositions as hereinbefore described.

The propellants of the present invention are, as previously stated, useful in the production of the impelling force for jet propulsion motors. The invention thus pro vides novel solids combining fuel and all the elements required for its combustion which can be used without exploding but with the production of great power.

These propellants are especially suited for use in rocket jet engines, which ordinarily comprise a combustion chamber where the fuel is combusted and one or more exhaust nozzles leading from the chamber to the atmosphere. Use of the self-combustible compositions of the present invention as charges in such motors is advantageous in that storage and feed systems for an oxidizing element are eliminated, with subsequent reduction of weight, a matter of great importance in aircraft. As a consequence of the saving in weight, a great gain in the ratio of total impulse to total weight is also realized. The substances are moreover relatively stable under a variety of conditions and hence safer than many compositions heretofore proposed, while at the same time being capable of generating great'power upon decomposition.

The nitroplastic propellants will not spontaneously ignite in a cool motor which allows a highly desirable safety factor. Accordingly, some means should be associated with the combustion chamber for ignition of the charge therein. Such suitable ignition or starting device may be a heating element located at the periphery of the combustion chamber, or some other ignition mechanism, such as an electric are, or an auxiliary flame introduced at a suitable place in the combustion chamber and caused to operate at the moment of starting. Such rocket jet engines are known in the art, as are suitable firing or ignition mechanisms usable therein. The propellant is merely secured in place in the combustion chamber, the ignition mechanism actuated and the propelled vehicle launched and/or maintained in motion by development of thrust by decomposition of the propellant. Numerous other advantages of operation and result accrue to the use of these novel propellants, such as simplicity of construction and operation of the jet motor, predetermined constancy of available energy, noncorrosive eifects on equipment, higher specific impulse with relatively low combustion and exhaust temperatures, and the like, additional advantages being immediately apparent to one skilled in the art.

References Cited in the file of this patent UNITED STATES PATENTS 2,277,083 Dorough Mar. 24, 1942 2,287,093 Ellis June 23, 1942 2,310,943 Dorough Feb. 16, 1943 2,325,064 Lawrence July 27, 1943 2,400,806 Bruson May 21, 1946 2,404,688 Bruson July 23, 1946 2,407,131 Bruson Sept. 3, 1946 2,419,043 Urbanski Apr. 15, 1947 FOREIGN PATENTS 856,335 France Mar. 18, 1940 512,987 Great Britain Oct. 2, 1939 

1. A NITRO ALDEHYDE CONDENSATION PRODUCT OF POLYVINYL ALCOHOL WITH A COMPOUND SELECTED FROM THE GROUP CONSISTING OF TRINITROBENZALDEHYDE, 5-NITROFURFURAL AND 2,3,3TRINITROPROPANAL. 